1. Field of the Invention
The present invention relates to a thin-film ND filter and a manufacturing method of the ND filter. The neutral density (ND) filter is used as a light amount restrictor for a purpose of evenly attenuating a transmitted light amount over a whole visible range.
2. Description of the Related Art
In a photographing system, under an excessively high luminance of an object, there may be a case of over exposure where an excessive amount of light is incident upon a photosensitive surface even with a minimum aperture diameter obtained by a diaphragm. In cope with such a case, an ND filter is often attached to a part of the photographing system so as to regulate an incident light amount upon the photosensitive surface. In this case, for a spectral property of the ND filter, the incident light amount must be reduced evenly throughout the visible wavelength range, hence it is necessary to secure a uniform transmittance over the whole visible range. In the photographing system such as a camera and video, a plastic film base ND filter is in heavy use for a purpose of evenly attenuating the light amount over the whole visible range. The ND filter is roughly divided into two types: a first type obtained by applying or incorporating carbon particles, dye, and the like into a plastic film; and a second type obtained by forming a thin film of ND filter on the plastic film.
For the first type in which the carbon particles, dye, and the like are applied or incorporated into the plastic film, the filter surface has an insufficient film hardness and is easily damaged. Moreover, when the carbon particles, dye, and like are incorporated, films having a high water absorbing property are used such as triacetate cellulose or triacetyl cellulose (TAC). Therefore, warp or strain is generated because of a change of environment. Such an ND filter is assembled into a shutter unit of a camera which has been more and more downsized. In this case, there is possibility that the filter contacts other constituting components during the driving of the shutter unit, and may break. Moreover, as a problem common to the first type, a uniform light amount attenuation, which has been required for an advanced photographing system in recent years, cannot be achieved in a wavelength range of about 400 to 650 nm for use in the photographing system, and an additional step of adjusting a color balance may be performed for compensating the deficiency of the ND filter. This causes a problem of manufacturing cost increase.
For the second type in which the ND filter is formed by depositing a thin film on the plastic film, a metal material is used as a light absorbing thin film as disclosed in Japanese Patent Application Laid-Open Nos. 1982-195207 and 1986-183604. Otherwise, a metal oxide film is used as disclosed in Japanese Patent Application Laid-Open Nos. 1989-51347 and 1995-63915. When the metal material is used as the absorbing film, the metal film has a thickness of 10 nm or less and is very thin, and it is difficult to control the film thickness for obtaining target optical properties. Furthermore, when the ND filter is placed under an ordinary environment for a long time after the film formation, oxidation of the metal film causes phenomena of transmittance increase or transmittance flatness deterioration in the ND filter with respect to the wavelength of the incident light. A countermeasure to such a deficiency may include sealing the metal film with an adhesive in order to suppress the deterioration of the film with an elapse of time, and using cryolite to laminate the film. This countermeasure increases the number of fabrication steps and disadvantageously thickens the film. On the other hand, when metal oxide is used in the light absorbing film, in general the metal oxide material is used as an evaporation source and deposited into the absorption film on the plastic film by vacuum evaporation. In this case, there is an advantage that film hardness increases. However, since it is difficult to adjust a whole film stress, a crack is generated in the film surface or the thin film peels. Moreover, the film is hard, and therefore a problem occurs that the film surface breaks or cracks during the process of shaping the film into a ND filter product.
The present invention has been developed in consideration of the above-described prior-art problems, and an object thereof is to provide a thin-film ND filter in which a light amount can uniformly be attenuated throughout the visible range and which is superior in durability. To achieve the object, according to the present invention, there is provided a neutral density filter for absorbing an incident light. The ND filter comprises a substrate made of a transparent material, and a multi-layer structure of thin films laminated on the substrate, the multi-layer structure containing a transparent thin film composed of a dielectric material effective to transmit the incident light and a non-transparent thin film effective to absorb a part of the incident light, wherein the non-transparent thin film is composed of a metal material which is deposited by vacuum evaporation, the non-transparent thin film containing an oxide of the metal material, which is generated during the vacuum evaporation by introducing a gas containing an oxygen while maintaining a vacuum degree in a range of 1xc3x9710xe2x88x923Pa to 1xc3x9710xe2x88x922Pa, and which is deposited concurrently with the metal material.
Preferably, the transparent thin film and the non-transparent thin film are successively laminated on the substrate to form the multi-layer structure, and then the multi-layer structure is annealed under an oxidizing atmosphere containing 10% or more of oxygen so as to saturate optical properties of the multi-layer structure. The metal material of the non-transparent thin film is selected from a group consisting of Ti, Cr and Ni, and the dielectric material of the transparent thin film is selected from a group consisting of SiO2 and Al2O3. Expediently, the transparent thin film and the non-transparent thin film are successively laminated in a predetermined order at a predetermined thickness to form the multi-layer structure effective to prevent reflection of the incident light.
In the present invention, the ND filter is formed by the thin film technology. This can inhibit the generation of damage because of insufficient surface hardness, which has been a defect of the conventional type. Moreover, when a plastic film substrate having a high durability is used as the transparent substrate, shape changes such as warp and strain can be suppressed. Furthermore, in the present invention, the metal is used as the raw material for forming the light absorbing film, and the mix gas containing oxygen is introduced during the film formation. When the partial pressure of oxygen and the total pressure of the mix gas are optimized, the metal oxide can be added into the metal film with a good reproducibility. As compared with the absorbing film formed only of the metal material, an absorption coefficient can be reduced by the introduction of the metal oxide. The film thickness is increased, and it is therefore easy to control the film thickness in order to obtain the target optical properties. To suppress the change of the optical properties with an elapse of time which is one of problems in using the metal material for the light absorbing film, the metal material is subjected to a heat treatment or annealing in an oxidization atmosphere containing oxygen beforehand. The changes of the optical properties by the oxidation of the metal material are thus forcibly saturated, and an ND filter stable for a long time can be obtained. Additionally, when the heat treatment is performed, the optical properties certainly change. Unless some measure is taken, flatness of the absorption with respect to wavelength is possibly lost by the heat treatment. Then, the film is designed by counting a change amount by the heat treatment in a film formation stage beforehand, and thereby the satisfactory flatness of the transmission property after the heat treatment is realized. Concretely, a variation of transmission density can be controlled to xc2x10.01 or less in a wavelength range of 400 to 650 nm. Moreover, to prevent the crack or peel after the film formation which is one of problems in using the metal oxide for the light absorbing film, the dielectric film having a high adhesion such as SiO2 is used in a first layer on a substrate side. Additionally, the stress of the whole laminated films is adjusted, so that the ND filter indicating a satisfactory surface state can be obtained.